Copper (Cu) is emerging as the metal of choice in a wide variety of semiconductor applications. Lower electrical resistivity, coupled with improved electromigration performance and increased stress migration resistance are important material properties that favor the use of Cu over aluminum (Al) in interconnect lines and contacts. The lower electrical resistance is critical since it allows signals to move faster by reducing the RC time delay. The superior resistance to electromigration, a common reliability problem in Al lines, means that Cu can handle higher power densities. An equally important benefit of Cu over Al is that the manufacturing cost for a Cu metallization scheme can be lower due to new processing methods that reduce the number of manufacturing steps and alleviate the need for some of the most difficult steps.
However, the introduction of Cu into multilevel metallization architecture requires new processing methods for Cu patterning. Because Cu is difficult to dry etch, new process schemes have been developed for Cu patterning. The damascene approach is based on etching features in the dielectric material, filling them with Cu metal, and planarizing the top surface by chemical mechanical polishing (CMP). Dual damascene schemes integrate both the contacts and the interconnect lines into a single processing scheme. However, Cu CMP technology is challenging and it has difficulty defining extremely fine features.
An alternative to the damascene approach is a patterned etching of a Cu layer. The patterned etch process involves deposition of a Cu layer on a substrate; the use of a patterned hard mask or photoresist over the Cu layer; patterned etching of the Cu layer using a reactive ion etching (RIE) process; and deposition of dielectric material over the patterned Cu layer. Patterned etching of Cu can have advantages over damascene processes since it is easier to etch fine Cu patterns and then deposit a dielectric layer onto the Cu pattern, than it is to get barrier layer materials and Cu metal to adequately fill small feature openings in a dielectric film.
Magnetoresistive random access memory (MRAM), is an example of new memory technology that can benefit from new dry etching methods for patterning Cu layers with magnetic materials. The lack of suitable fast etching processes for materials such as Cu and various magnetic materials, can limit the ability to batch-fabricate sub-micron magnetic devices. Magnetic materials, that contain transition metals such as Ni, Fe and Co, are substantially inert in conventional dry etch processes. Patterning of magnetic devices has been predominantly accomplished using Ar+-ion milling or additive deposition processes such as electroplating or lift-off, but these methods are undesirable for semiconductor batch processing.
The primary etch gas for etching Al and Cu layers is traditionally a chlorine-containing gas in a gas mixture that includes argon (Ar). The chlorine-containing gas is selected from a large group of chlorine compounds such as Cl2, HCl, BCl3, SiCl4, CHCl3, CCl4, and combinations thereof. To achieve anisotropic etching, Cl2 is mixed with other chlorine-containing gases that are selected from the above list, since the use of Cl2 alone results in isotropic etching.
Etching of Cu layers using chlorine plasma essentially involves physical sputtering of the CuClx layer by energetic ions in the plasma. The etching rates with this method are very low and another drawback is that the sputtered CUClx coats the chamber walls and this requires periodic cleaning of the chamber. An equally serious problem is encountered when high-aspect-ratio features are etched in chlorine plasma and the sputtered CuClx products redeposit on the feature sidewalls where the effects of physical sputtering are reduced.
Furthermore, when the process is carried out at elevated temperatures (>200° C.) to increase the volatility of the reacted Cu layer, corrosion can occur due to accumulated CuClx etch residues on the surface. If these residues are not removed by a post-etch cleaning step, they can cause continuing corrosion of the Cu even after the application of a protective layer over the etched features.
Other approaches for dry etching of Cu that involve copper halides have been examined to try to accomplish higher Cu etch rates. In addition to high processing temperature, the use of additional energy sources, such as exposure of the etch surface to UV or IR light to accelerate the desorption of CuClx have been proposed. These alternative approaches are not practical for semiconductor batch processing of large substrates due to poor etch uniformity, high cost and added equipment complexity, and reliability problems.
Nelson in U.S. Pat. No. 4,468,284 entitled “Process for etching an aluminum-copper alloy,” describes a process for etching Al—Cu alloys that contain up to 6% Cu by weight. The plasma process comprises reactive chlorine and a NO+ species that aids in the oxidation of Cu to CuCl2. An Al2Cl6 reactant is formed in-situ from the etching of Al in the Al—Cu alloy and it reacts (complexes) with the surface CuCl2 to form a volatile CuCl2—Al2Cl6 complex that is removed from the etching surface.
Bausmith et al. in U.S. Pat. No. 4,919,750 entitled “Etching metal films with complexing plasma,” describes a method for dry etching metals that form low-volatility chlorides. The method involves exposing a layer of metal to a chlorine-based plasma in presence of a metal source spaced apart from the workpiece. When reacted with the plasma, the metal source provides gaseous metal-containing reactants that serve as complexing agents with the metals in the surface of the workpiece to be etched. The workpiece can comprise Co, Cu or Ni metal films and the metal source is selected from Al, Ga, Fe and In.
Both of the abovementioned patents involve forming etching reactants in-situ during the process, which can result in poor control over the delivery of the etching reactant to the surface of the metal film to be etched. Therefore, to provide better etch control and better control over reactant delivery; it is desirable to introduce gaseous etching reactants from ex-situ sources such as gas cylinders or precursor containers.
Due to the introduction of Cu in new and existing thin-film technologies, there is a need for dry etching methods that allow etching and patterning of pure Cu layers and Cu-containing layers, using gaseous reactants that form volatile Cu-containing etch products.